Because the variables have ranges to work from rather than definitive figures. If you think of all the difference risks you put on the form, your risk of claiming will be say, £2,000-£2,500, not £2,205.17. All the form is really doing is narrowing down the range of what you'll be charged.
It's not definitive, insurance never can be because they never know what you will do, they will just base the likelihood of what you will do on statistics, the accuracy of the estimates will vary because of this. It's the only real way they can do it, looking at each person individually will only increase prices anyway, because underwriting costs so much more in that instance.
As Famine says, the main problems are Premiums are higher than they should be, due to uninsured drivers (which quite frankly is a stupid situation, if you don't pay anyone to indemnify you on the road then you should be forced by the courts to financially pay for any claim by any party involved), the injury claim culture and those who are higher risks and do actually cause severe injuries and fatalities not actually paying enough.
Also, looking at your experience, you say one was done online and the other over the phone. Did he do the online quote through a comparison site? Because they can be out of date. How different was the quote?
There's also the fact from a sales perspective, it's easier to sell to someone over the phone than it is through a website, so that will create a difference in some instances and depends who you speak to as well, different people will have different authority (Those who don't notice a difference will be invariably those who don't do research or get quotes elsewhere and use that to their advantage to pull the price down.)
That's how I've understood it from a friend who is a broker for one of the main insurers. In fact it was he who dissuaded me from doing Pass Plus as it was only brought in to bring some insurers in-line with other insurers.